(A comment, not a question; I made a mistake in my translation, and I now see why I went wrong) >
I had to scratch my head a bit to see why there is no tilde in 'cuando' in the sentence [in the text]:
"Aún recuerdo cuando teníamos que revelar los carretes" [= I still remember when we had to develop rolls of film ...]
-- particularly after noticing the example: "No recuerdo cómo tomas el té ..." [= I can't remember how you take your tea ...] -- in: Difference between cómo and como in Spanish (with and without an accent)
I now realise that, in order to carry the tilde, the 'cuándo' or the 'cómo' must be part of an indirect question - which is indeed the case with the second example, but not the first.
Inma, Shui and Silvia - you do indeed provide us with interesting and useful exercises and explanations: Keep up the good work!